This invention relates to printed circuit board holders and more particularly to a holder and module comprised of said holder and board adapted to transfer heat from circuit components mounted on the printed circuit board to a heat sink.
Certain printed circuit board components, specifically, integrated circuit chips of the dual-in-line pin (DIP) type, are limited in the maximum temperature in which they are capable of operating. This operating temperature is determined by the heat dissipated within the integrated circuit element itself, the heat dissipated in other heat-producing elements including other integrated circuit chips, and the heat dissipating properties of the printed circuit board holder which is in thermal contact with the integrated circuit chip and a heat sink.
It is generally desired that the volume occupied by the electrical circuitry including the holder be minimized thereby resulting in the increase in the heat generated per unit volume or area. It is therefore necessary that the holder for the printed circuit board be properly configured to effectively carry away the heat generated from the integrated circuit chips to a heat sink by providing a low thermal impedance heat path.
The features by which the improvement in heat dissipation by the holder of this invention is achieved are made more evident by a description of the printed circuit board holders which had been available heretofore, together with their performance characteristics. FIG. 1 shows a prior-art printed circuit board holder 10, together with an exploded view of the printed circuit board 18 and an illustrative DIP integrated circuit 21 which is to be soldered to connector holes 19 on the printed circuit board 18. The holder 10 comprises metal frame 11 having a plastic pin connector 14 at one end. The connector 14 has electrically conductive pins 15 which are connected to respective pins (not shown) on the far side of the connector 14. These far side pins correspond with connector holes 20 of printed circuit board 18. The holes 20 are connected by printed circuit wiring (not shown) to holes 19 connected to DIP integrated circuits 21 or to resistor or capacitor components (not shown) which also may be mounted on the circuit board 18, thereby providing electrical connection between the pins 15 and the electrical components on the circuit board. The plastic connector body 14 has embedded in it metal pin shields 141. Guide pins 16 are also imbedded into the ends of the plastic connector body 14. The metal frame 11, electrically insulating solder mask 17, and the printed circuit board 18 are assembled with the rivets 22. The frame 11 and the mask 17 have slots 12 and webs 13. The DIP circuit 21 has connector pins 211 which straddle the webs 13 and which project through slots 12 into holes 19 of the printed circuit board 18. The underside 212 of the DIP circuit 21 makes thermal contact to the web 13 of frame 11 through a thermally conductive electrically-insulating tape 23. After the assembly as above described, electrical connection of the pins 15 211 is made to the printed circuit board 18 by wave soldering techniques.
A module 1 assembled as shown in FIG. 1 is capable of dissipating a total of 4.5 watts produced by circuit elements such as DIP package 21 while limiting the temperature rise of the circuit elements to 60.degree. above the temperature of a heat sink (not shown) such as a cooling fluid in spring finger contact with the edges 24 of frame 11. The heat sink forms a part of the mechanical and electrical configuration into which the module 1 of FIG. 1 is inserted. Although the thermal dissipation properties of the module 1 are satisfactory, the module 1 is less desirable from a production standpoint. Because of the design of module 1, assembly of its many parts requires hand assembly procedures and is not amenable to automation and thereby the module 1 is costly.
As stated earlier, assembly of module 1 requires the printed wiring board 18 must be solder masked by an electrical insulator 17 inserted between the printed wiring board 18 and the frame 11. The board 18 is then riveted to the frame 11 by rivets 22. Next, the thermally conductive tape 23 and the DIP circuits 21 are placed in position on frame 11 and the wave soldering of the preceding paragraphs completes the assembly. During the soldering process and when the module is in use, the pin shields 141 have a tendency to fall off the connector 14. In addition, the connector 14, being of plastic, requires that the keying pins 16 be inserted using adhesive and held in a prescribed position while the adhesive sets. These keying pins 16 frequency become loose. Therefore, it is seen that the module 1 of FIG. 1 has substantial inadequacies from a fabrication and reliability standpoint.
For those circuit configurations where the total dissipation of the components mounted on the printed circuit board is one watt or less, sufficient power dissipation to limit the temperature rise to 60.degree. C. is obtained by mounting the circuit board 18 on the "open frame" type of holder 31 shown on FIG. 2. The circuit board 18 is shown with DIP circuits 21 and connector 14 placed within the holder 31 ready to be secured thereto by rivets 22. The assembly of the printed circuit board 18 and the holder 31 of FIG. 2 has several advantages over the module 1 of FIG. 1. The most significant advantage is that the printed circuit board 18 may be soldered to its DIP circuits 21 and to its connector 14 by conventional wave soldering techniques before the assembly is riveted to frame 31. The assembly process may be highly automated, thereby greatly reducing the cost of manufacture. The frame 31 has an integral pin shield 311 and, therefore, the pin shields cannot fall off as they may in the configuration of FIG. 1. The keying pins 16 are also secured to the metallic frame 31 by a press fit so that they also are very securely mounted to frame 31 in comparison with their imbedment in the plastic connector 14 of FIG. 1.
Despite the mechanical advantage and ease of fabrication, the assembly of the frame 31 and the circuit board 18 of FIG. 2 has the limitation that the total allowance power dissipation of the DIP circuits 21 for the allowed 60.degree. C. temperature rise is limited to one watt. Since many circuit configurations exist which exceed one watt dissipation, it was necessary to resort to the more expensive assembly configuration of module 1 of FIG. 1 in those circumstances. In order to increase the power dissipation capability of the circuit board 18-frame 31 assembly, a cover plate 32 was attached to the assembly by the rivets 22. The cover plate 32 was placed in thermal contact with the DIP circuits 21 on the printed circuit board 18 by using a thermally conductive substance (not shown) between the components 21 and the cover plate 32. The electrically conductive material was also placed between the overlapping regions 33', 33" of the cover plate 32 and the frame 31. Because of the constraints on the size of the frame 31 and the area occupied by the DIP circuits 21, the available area of contact of the cover plate 32 and the frame 31 was only along their edges. Since the width of the regions 33', 33" on frame 31 is only appropriately 0.050 inch, the total area available for transfer of heat from the cover plate 32 to the frame 31 was not substantial, and the total power which could be dissipated for a 60.degree. C. temperature rise using the cover plate 32 was only about 1.5 watts, only a slight improvement over the 1 watt power dissipation without the cover plate 32 for the same temperature rise.